1. Field of the Invention
The present invention relates to an optical writing device, an image forming device, and a method of controlling the same.
2. Description of the Related Art
In recent years, computerization of information is getting increased. An image processing device, for example, a scanner to transform document into information with an electronic form and a printer or a facsimile to output the information are well known. Such image processing devices is usually an MFP, which are provided with an image-capturing function, an image-forming function, a communication function, and the like, used as a printer, a facsimile, a scanner, and a copier.
Among image processing devices, an electro-photography image forming apparatus is widely used as an image forming apparatus. In an electro-photography image forming device, an electrostatic latent image is formed by exposing a photosensitive element. A toner image is formed by using a developing agent such as toner for the electrostatic latent image. Finally, the toner image is transferred onto a sheet.
In such electro-photography image forming device, an image to be output is divided into multiple lines, and image formation is performed for each line. Generally, an optical writing device, in which light-emitting elements corresponding to pixels are arranged for a main scanning line, exposes the photosensitive element for each line, thereby forming an electrostatic latent image. An LPH (Light-emitting diode Print Head) may be used as a light source for such optical writing device.
Unfortunately, the conventional optical writing device such as the LPH has an error associated with attaching to the image forming apparatus. The LPH is mechanically attached to the image forming apparatus. There are the cases where the LPH is attached to the image forming devise in an inclined manner relative to the defined position, and the attachment position is inclined due to operation vibration, temperature variation, or change over time. These are generally referred to as “skew”. When the LPH inclines from the defined position, the output light of the light-emitting element is not accurately emitted onto an image-forming position on the photosensitive element exposed by the LPH. This leads to a degradation of a quality of the image, and a color deviation among colors of C (Cyan), M (Magenta), Y (Yellow), and K (black).
To solve the above disadvantages, Japanese Patent Laid-Open Publication No. 2012-061675 discloses a technique in which multiple main scanning division positions are set in advance and an image is shifted in the sub-scanning direction at the division position to correct the inclination. The image is shifted by shifting a line which is read at the division position when reading pixel information stored in a line memory for storing the pixel information for each main scanning line.
A generally-available LPH is constituted by implementing multiple semiconductor chips each including multiple integrated light-emitting elements on a substrate so as to satisfy the width in the main scanning direction. For example, light-emitting elements corresponding to 192 dots are integrated into a semiconductor chip at a pitch of 42.3 μm, and 26 pieces of semiconductor chips are implemented on a substrate to achieve a resolution of 600 dpi for A4 size.
As described above, in the LPH thus constituted, multiple light-emitting elements integrated into one semiconductor chip, and therefore, positional deviation between light-emitting elements in the semiconductor chip would not cause any problem. When multiple semiconductor chips are implemented on a substrate, positional deviation between semiconductor chips may occur. Therefore, not only the inclination of attachment of the LPH relative to the image forming device but also positional deviation between semiconductor chips in the LPH may cause problems such as degradation of the image quality, color deviation, and the like described above.
Even if such positional deviation between semiconductor chips in the LPH is corrected, inclination correction using shifting of the image as described above can be used. More specifically, on the basis of information indicating the state of positional deviation between semiconductor chips, the image is shifted in the sub-scanning direction at the division positions as described above. Such correction of the positional deviation between semiconductor chips is called waviness correction.
In this case, when both of the skew correction described above and the waviness correction are performed, the correction values of them both may overlap, and the correction amount, i.e., the shift amount, in the sub-scanning direction at a division position may increase. When the pixel information is read from the line memory and the correction amount in the sub-scanning direction is informed to a module controlling the light source, this is performed with a register provided for each of the division positions described above.
In this case, much correction amount is designated at a division position, and therefore, each of the registers is required to have many numbers of bits, and this leads to increase of the circuit scale and the production cost. When the correction amount at a division position is much, the image may be deformed in the portion. Therefore, the shift amount at a division position is desirably reduced to the minimum level as much as possible.
The problem as described above is not limited to the use of the LPH as the light source. The same problem would occur even when other type of light source is used to constitute a light source on a line.
Therefore, there is a need for reducing the shift amount at a shift position in an optical writing device that performs skew correction and waviness correction by shifting an image in a sub-scanning direction.